debug .env file

There seem to be some issues with nested environment variable definitions in the .env file (which depend on the Docker version?). E.g. in docker-compose.yml, many containers have environment variables depending on $E_APP_BASE_URL but it does not get provided to the container unless specifically specified.

Definition of Done:

• look up in documentation or forums
• fix it
• test
• mention guidelines to colleagues --> nothing to mention except "test it locally before you commit"

Hints:

• Docker documentation probably has answer to this issue...